The invention relates to a continuous mixer. In another aspect, the invention relates to a process for melting a particulate material.
The advantages of continuous processes as compared to batch processes in industries characterized by high throughput are well known. One industry in which continuous processes have gained wide acceptance is the polymer industry. This is especially true for the polyolefins and polyvinyl chloride (PVC) industries, largely because of the high throughput of commercial plants.
Generally, polymers are recovered from their formation processes in the form of a fluff of very fine particles which, because of its low bulk density, is unsuitable for shipment to manufacturers. It must be densified to a high apparent bulk density. The process of densifying the polymer usually involves melting it and forming it into pellets. Since the polymer must be melted in order for it to be formed into pellets, the opportunity is presented to compound certain desirable agents into the polymer. Thus, when the polymer is melted it is usually simultaneously compounded with pigments, reinforcing agents, stabilizers, processing aids and the like prior to its being pelleted. It is to this process of melting the polymer and optionally mixing it with a desired ingredient to a point of homogeneity that the present invention is addressed.
One type of mixer which has gained wide acceptance in the industry is denoted as a Farrel Continuous Mixer (FCM) which is sold by the Farrel Corporation of Ansonia, Conn. This machine is characterized by parallel twin rotors enclosed in a suitable housing or barrel with a feed entrance at one end and a melt exit at the other end. The device is manufactured in various sizes which are normally characterized by rotor diameter. For example, an FCM with a 2-inch diameter rotor is commonly denoted a 2FCM and a mixer with a 9-inch diameter rotor is denoted a 9FCM which is a commercial device. The barrel length is generally a multiple of the rotor diameter and is usually about 3.5 times the rotor diameter. The device is compact and capable of high throughput.
The FCM is available with various rotor designs. Naturally, certain designs are recommended by the manufacturer for certain applications. The rotor style recommended primarily for polyolefins is denoted a #15 rotor style. A #7 rotor style is recommended for processing polyvinyl chloride. The #7 rotor is a milder design that the #15 and does not mix as vigorously. When the #7 rotor style is used for polyolefin processing the ribbon temperature and specific energy requirements of the melting and mixing process are lowered but mixer process control is unstable.
It would be desirable to provide a process for melting and mixing plastics, especially polyolefins, in which the maximum temperature to which the polymer is subjected is as low as possible provided that adequate mixing and melting is achieved. It would be further desirable to provide a process and apparatus for melting and mixing polyolefin polymer, especially polyethylene, which is characterized by low specific energy requirements, defined as the energy expended per pound of polymer processed, provided that adequate mixing and melting of the polymer is achieved. It would be still further desirable to provide a mixer and a mixing process characterized by outstanding stability against changes in feed rate.